1. Field of the Invention
The present invention relates to a calculus destroying apparatus for destroying a calculus by using a shock wave and, more particularly, to a calculus destroying apparatus using piezoelectric elements as a shock wave source.
2. Description of the Related Art
Recently, as a method of removing a calculus such as a renal calculus without performing an operation, a method of destroying a calculus by focusing a shock wave onto a calculus portion in a body from an external shock wave source has been proposed and widely used. This method less invades a patient, i.e., a living body than an operation. If, however, a focal point of a shock wave falls outside a calculus portion and the focused shook wave is radiated onto a normal tissue, a side effect on the normal tissue cannot be prevented. Actually, a calculus portion often falls outside a focal point of a shock wave source due to a respiratory motion or a body motion of a patient.
A calculus destroying apparatus which copes with the above problem is disclosed in U.S. Pat. No. 4,803,995. This calculus destroying apparatus employs piezoelectric elements as a shock wave source and operates such that a low pressure ultrasonic wave in the same focused state as a shock wave is radiated in a body via the piezoelectric elements for calculus destruction and the coincidence between a focal point of the shock wave source and a calculus portion is determined in accordance with the intensity of an echo from a portion near the focal point. According to this apparatus, since a shock wave is radiated only when a focal point and a calculus portion coincide with each other, erroneous radiation caused by, e.g., a respiratory motion can be prevented.
Generally, piezoelectric elements used to generate a shock wave for calculus destruction are accurately concavely arranged to form a pressure distribution extremely focused to have a half amplitude width, i.e., half width, in the radial direction of about 2 to 4 mm so that a very high pressure of about 600 kbar to 1 kbar is obtained at a focal point. A low pressure ultrasonic wave focused to the same degree as this calculus destroying shock wave was radiated on a spherical target shown in FIG. 1A as a model calculus, and a peak value distribution of its echo intensity was measured. The measurement result is shown in FIG. 1B. An active alumina sphere having a diameter of 7 mm was used as the spherical target. Referring to FIG. 1B, peak values of the echo intensity obtained when the spherical target was moved in the direction of depth of the focal point and perpendicularly to a central axis are plotted. In this case, the half width of the echo intensity distribution shown in FIG. 1B becomes 4 mm which is 57% of the diameter of the target.
Assume that in the above conventional technique, when the echo intensity reaches the intensity of the half value, i.e., 1/2 or more the maximum intensity, it is determined that a focal point of a shock wave source coincides with a calculus portion, and a shock wave is radiated. In this case, it is expected that the shock wave can be radiated on a portion only about 57% the diameter of a calculus but cannot be radiated on its end portion.